When auditory hair cells (HCs) are lost, they are not replaced in humans or other mature mammals, resulting in permanent hearing loss. In contrast, regeneration of auditory HCs naturally occurs in non-mammals, allowing for recovery of hearing function. We have recently observed that the neonatal mouse cochlea can spontaneously regenerate its HCs after damage; however, the majority of these regenerated cells died. In addition, many studies have shown that outer HCs (OHCs) are more susceptible than inner HCs (IHCs) to damage caused by noise or ototoxic drugs. Yet the mechanism that makes OHCs more vulnerable is not understood. There is also a lack of understanding of the pathways that regulate HC survival under normal conditions after differentiation is complete. Proposed studies will investigate HC survival during postnatal maturation and adulthood, during HC regeneration, and in stressed HCs following noise exposure. We will focus on one gene, Pou4f3, a transcription factor that is expressed in HCs beginning with differentiation. While the role of Pou4f3 in maintaining HC survival during development was discovered previously, its role in postnatal maturation, aging, regeneration, and in stressed HCs has not been explored. Preliminary data show that deletion of Pou4f3 in adult OHCs causes cell death which suggests that mature OHCs still require Pou4f3 expression to survive. In addition, POU4F3 immunostaining results show that many adult OHCs have decreased levels of POU4F3 expression, while IHCs retained strong POU4F3 expression. These data suggest that complete deletion of POU4F3 causes HCs to die, but reduced levels of POU4F3 are enough to maintain HC survival. Aim 1 will investigate this further by deleting Pou4f3 in neonatal, juvenile, and adul HCs. In addition, the majority of regenerated HCs that spontaneously form in the neonatal mouse cochlea do not express POU4F3 and we hypothesize that this causes cell death. In support of this hypothesis, another study ectopically expressed Atoh1 in supporting cells to convert them into HCs. These newly formed HCs survived at least 3 months and did express POU4F3. These data implicate Pou4f3 in the regulation of HC survival during the differentiation process and Aim 2 will rescue regenerated HCs in the neonatal mouse cochlea by ectopic expression of Pou4f3. Since Pou4f3 is known to regulate several genes involved in apoptosis, decreased levels of POU4F3 expression in OHCs of adult mice may make these cells more vulnerable under stressful conditions and account for the increased damage susceptibility of OHCs. We will test this hypothesis in Aim 3 by over-expressing Pou4f3 to protect OHCs from noise-induced damage. Collectively proposed studies will investigate Pou4f3's role in the regulation of HC survival during maturation and adulthood in the normal, undamaged cochlea, during spontaneous HC regeneration in the neonatal mouse cochlea, and in stressed HCs following noise exposure. Completion of these aims will advance our understanding of the mechanisms that regulate HC survival under multiple conditions, which could be used to develop drugs to protect HCs.